BY MAHJORIE ISABEL COLLINS. 



2oo 



scribed, such as species of BrachylcEua, Baccharis, EscallonKt, etc., 

 the stomata are belated in development: and, in a few cases in 

 which the secretion remains active until maturity of the leaf, the 

 stomata become elevated until they project above the surface of 

 the secretion. The mature stoma of .V. crassifolia remains at 

 the surface-level of the leaf. There is little of interest to remark 

 in the structure of the mature stoma, a small, outer chamber 

 being formed in the usual way, by an overhanging lip of cuticle. 

 Internal Development of the Leaf. 



As in the case of the stomata, the development of the leaf- 

 tissues is belated. In a young leaf just freed from the bud, there 

 is no internal differentiation 

 except that of the vascular sys- 

 tem. The leaf, at this stage, is 

 made up of a compact mass of 

 rounded cells, the vascular 

 strands occupying a median 

 position. It is only when the 

 secretion becomes less active, 

 that differentiation of the leaf- 

 tissues takes place. 



The fully-grown leaf of .S'. 

 crafisi/olia is very thick and 

 succulent, owing to the devel- 

 opment of secondary, water- 

 storage parenchyma. The epi- 

 dermal cell-walls show a great 

 development of cellulose, but 

 are only moderately cuticular- 

 ised. There is no si^ongy meso" 

 phyll, the greater part of the 

 leaf being made up of several 



rows of palisade-cells. A tis- 

 sue of rounded cells, amongst 

 which are situated the vascular 

 strands, occupies a median posi- 

 tion in the leaf, and is bounded on either side by, sometimes, as 



Text -tig. o. — Section of mature leaf 

 showing occurrence of branched 

 nnicilage-cells in the palisade- 

 tissue; ( X 300). 



